Lysozyme is increased in inflammatory reactions and is a component of the extracellular matrix, but its possible role in lung diseases such as emphysema and interstitial fibrosis has not been investigated. Determining the significance of any changes in pulmonary lysozyme content is complicated by the fact that this protein has no recognized physiological function in the lung other than protecting it from bacterial infection (1-3).
To further understand the role of lysozyme in pulmonary disease, tissue sections from normal, fibrotic, and emphysematous human lungs were evaluated for differences in lysozyme content. An increase in extracellular lysozyme was specifically observed in lung tissues with pulmonary emphysema, and the protein was preferentially associated with elastic fibers, which undergo breakdown in this disease (4).
Since this laboratory and other investigators have previously shown that hyaluronan and other polysaccharides surround elastic fibers (5-7), normal lung tissues were treated with hyaluronidase and examined for their ability to bind exogenously administered lysozyme. Such treatment resulted in increased attachment of lysozyme (4), suggesting that degradation of extracellular matrix components, as occurs in pulmonary emphysema, may expose binding sites for lysozyme on elastic fibers. In vitro studies, using an extracellular matrix preparation mainly composed of elastic fibers, confirmed that lysozyme has a strong affinity for these fibers (unpublished observations).
While the mechanism responsible for the observed affinity of lysozyme for elastic fibers is unclear, it is possible that lysozyme may bind to specific carbohydrate residues in elastic fibers. N-acetyl-D-glucosamine, a component of bacterial cells susceptible to degradation by lysozyme, has also been found in glycoproteins associated with elastic fibers (8). Injury to elastic fibers, as occurs in pulmonary emphysema, may expose such residues, thereby facilitating lysozyme binding.
The enhanced binding of lysozyme to elastic fibers in pulmonary emphysema may protect these fibers from further injury. Previous work by other investigators has shown that lysozyme prevents elastolysis in vitro (9). Lysozyme could therefore be useful in treating emphysema and other diseases involving damage to elastic fibers, such as asthma, pulmonary fibrosis, respiratory distress syndrome, bronchopulmonary dysplasia, and cystic fibrosis. This protective effect of lysozyme would complement its antibacterial properties (1-3) and make it particularly beneficial in the treatment of certain types of pulmonary infections where there is necrotizing lung injury. Similarly, lysozyme has been reported to counteract HIV infection (10) and may therefore be useful in the treatment of pneumonias and other disorders associated with AIDS.
Another useful property of lysozyme is its ability to bind to and disaggregate hyaluronan and other polyanionic compounds (11). Lysozyme might therefore be utilized to treat lung diseases involving excess mucus secretion in airways. In particular, this protein may help alleviate the obstruction of airways associated with pneumonias, asthma, and cystic fibrosis.
This same ability of lysozyme to disaggregate hyaluronan may also be beneficial in pulmonary fibrosis, where significant accumulation of this polysaccharide occurs in combination with collagen, elastin and other polysaccharides (12-14). By disaggregating hyaluronan, lysozyme may interfere with the fibrotic process, thereby ameliorating the disease. As shown in studies from this laboratory (4), there is a decrease in lung lysozyme content in pulmonary fibrosis (relative to the proliferation of other tissue components), which may conceivably facilitate the fibrotic response.
With regard to intratracheal administration of lysozyme, this laboratory has shown that an aerosol preparation of the protein rapidly penetrates the lung, remains there for at least 24 hrs, and does not cause pulmonary injury (15). These findings suggest that lysozyme could also act as a vehicle for intratracheal delivery of drugs for the treatment of pulmonary and systemic diseases. By virtue of its attachment to elastic fibers, lysozyme could slow the pulmonary clearance of inhaled therapeutic agents, thereby increasing their effectiveness in the lung.
To date, the use of intratracheally instilled lysozyme as an adjunct to conventional antibiotic therapy has been limited to the administration of the antibiotic via routes other than intratracheal instillation (16). The disadvantages of this type of combined therapy are: 1) reduced concentration of the antibiotic in the lung due to systemic dilution; 2) the inability of lysozyme to interact with and potentiate maximal dosages of the antibiotics due to separate routes of administration (e.g. intratracheal lysozyme with oral carbenicillin); 3) different clearance rates of lysozyme and antibiotic from the lung, resulting in a loss of interaction time between the two agents.
In contrast, the combined intratratracheal administration of lysozyme with antibiotics (and other treatment agents) takes advantage of their common anatomical and temporal associations within the lung. This form of administration should maximize the effects of the particular therapeuctic agent combined with lysozyme. For example, intratracheal administration of lysozyme with an antibiotic such as rifampin (whose site of action is within the cytoplasm of the pathogenic organism) would allow more rapid penetration of the antibiotic into the organism due to the cell wall-lytic effect of lysozyme. Maximal synergy between the two agents is dependent on their presence in the lung at the same time and place, which is only possible by their combined intratracheal administration.
The novelty of this approach is made apparent by the fact that previous attempts to combine lysozyme with antibiotics were based solely on the potential for lysozyme to stimulate the systemic immune system, not potentiate the effect of the antibiotic per se. Therefore, the need to intratracheally administer lysozyme in combination with the antibiotic was not perceived as advantageous by one skilled in the art. Further evidence of the novelty of this approach is provided by the fact that such combined intratracheal administration of lysozyme and antibiotics (or other therapeuctic agents) has not been attempted despite the availability of lysozyme for therapeutic use over the past half-century.
The possibility that lysozyme may be effective against pathogenic organisms in vivo has only been described very recently, subsequent to the filing of the initial patent (U.S. Pat. No. 6,776,989) by the applicants on Apr. 5, 2001.